Add new run_jobs_synchronously! helper for tests

Previously if you wanted to have jobs execute in test mode, you'd have
to do `SiteSetting.queue_jobs = false`, because the opposite of queue
is to execute.

I found this very confusing, so I created a test helper called
`run_jobs_synchronously!` which is much more clear about what it does.

spec/requests/admin/groups_controller_spec.rb

@@ -78,7 +78,7 @@ RSpec.describe Admin::GroupsController do
     let(:user2) { Fabricate(:user, trust_level: 4) }
 
     it "can assign users to a group by email or username" do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
 
       put "/admin/groups/bulk.json", params: {
         group_id: group.id, users: [user.username.upcase, user2.email, 'doesnt_exist']